The present invention relates to stator vanes used to direct airflow between stages within a compressor of a gas turbine engine. More particularly, this invention relates to a method for increasing the reliability of stator vane structures by welding shims to a base of the stator vane.
A conventional gas turbine generally operates on the principle of compressing air within a compressor, and then delivering the compressed air to a combustion chamber where fuel is added to the air and ignited. Afterwards, the resulting combustion mixture is delivered to the turbine section of the engine, where a portion of the energy generated by the combustion process is extracted by a turbine to drive the compressor via a shaft.
In multi-stage compressor sections, stators are placed at the entrance and exit of the compressor section, as well as between each compressor stage, for purposes of properly directing the airflow to each successive compressor stage. As a result, the stators are able to enhance engine performance by appropriately influencing air flow and pressure within the compressor section.
Stators generally consist of an annular array of airfoils, or vanes. Stators are typically formed in segments as stator vane units consisting of one or more airfoils supported by the base. These stator vane units are then individually mounted to the compressor casing to form an annular array, so that the airfoils project radially between an adjacent pair of stages.
Stator vanes in an industrial gas turbine compressor are loaded and unloaded during start-stop cycles. In addition, the vanes are subject to small pressure fluctuations during operation. These result in relative motion between the vane base and the casing in which the vanes are assembled. The relative motion results in wear of both the vane base and casing, which, in turn, results in loose vanes. The loose vanes become more susceptible to relative motion and begin to chatter. Expensive repair or replacement of the vanes and casing does not solve the wear and chatter problem; it simply begins the process anew. Repair and/or replacement of the vanes and casing are expensive. Similar problems exist between stator ring segments, which hold a plurality of stator vane units, the stator ring segments being mounted in slots of the compressor casing.
FIG. 1 illustrates a compressor section 10 showing a portion of an open casing 15 of a compressor showing five exemplary stages (rows) 20a-20e of stator vane units 25. In the embodiment shown, the casing section 15 is semicircular. The casing 15 has a mounting surface 30 that may be secured to a corresponding mounting surface on another casing section with fasteners extending through a plurality of holes 35. For a complete compressor, two of the semicircular casing sections would be fitted together around a rotor (not shown).
Each stator vane unit 25 has the airfoil vane 40 that extends upwards from a base 45 and radially inward towards the shaft of the compressor rotor (not shown). The airfoil vanes 40, stator vanes, are interposed between the rotor blades (not shown). Certain stator stages of a compressor may mount stator vane units directly in a slot in the casing. Other stator stages mount stator vane units in ring segments, which are then mounted in slots of the casing. Both types of mounting will be described in more detail.
FIG. 2 illustrates individual stator vane units. Airfoil vane 40 extends vertically from a base 45. The base 45 has two opposing retaining faces 50. The base 45 has a pair of projections 55, one on each of the retaining faces. The projections 55 are to be received by a correspondingly shaped groove in a slot of the casing. The grooves retain the stator vane unit 25 in place in the slot of the casing. The other two opposing faces of the base 45 are the engaging faces 60. The engaging faces 60 of base 45 butt against the bases 45 of adjacent stator vane units when the units are installed in a casing slot.
FIG. 3 illustrates an enlarged side view of the casing showing a stage in which individual stator vane units are assembled in a slot of the compressor casing. For this type of installation, a plurality of the stator vane units are assembled in the casing to form the stator vane stage. The casing 15 has a plurality of slots 70 for receiving the stator vane units 25. The slot 70 has a pair of side edges 75, which each has a groove or dovetail-shaped recess 80. The square base dovetail 80 holds the vane units 25 in place. Each vane unit 25 is allowed to slide into place with the base 45 received in the slot 70 and the projections 55 received in the grooves 80. The casing 15 in the particular embodiment shown has the air extraction cavity 85 that underlies the stage and is formed by the slot 70 and the stator vane units 25. While a square base 45 for the vane unit 25 is shown, it is recognized that other shapes may be desired dependent on the number, size and shape of the airfoil. For example, the base 45 can have a rectangular shape or a parallelogram shape.
The stator vane units 25 for an individual stage are sequentially placed in the slot 70 of the casing 15 until the full circumferential run of the slot has been filled with a designated number of stator vane units.
Other stages of stator vanes may be attached to the casing using ring segment assemblies. The ring segment assembly includes a ring segment and a stator vane unit. Ring segments hold a plurality of stator vane units. After the ring segments have been loaded with stator vane units, the ring segments are slid into circumferential slots in the turbine casing and are butted against each other to sequentially fill the circumferential slots. Blades that are larger and have more forces placed on them may be assembled using this vane and ring segment assembly to provide a stiffer base mount.
FIG. 4 illustrates a ring segment assembly 85 that is slid out and away from the casing 15. The ring segment 90 receives a plurality of stator vane units 25. A base 45 of the stator vane units 25 slides (in a generally axial direction with respect to the compressor) into the ring segment 90. The base 45 of the stator vane unit 25 includes a dovetail 95 fitting into and being retained by a corresponding dovetail-shaped slot 100 in the ring segment 90.
The ring segment 90 slides into the circumferential slot 70 of the casing 15. The sidewalls 105 of the ring segment 90 are supported axially by the sidewalls 110 of the slot 70 when the ring segment 90 is within the slot 70. The square base dovetail 115 of the ring segment 90 fits into the grooves 120 of the circumferential slot 70, thereby retaining the ring segments 90 in the circumferential slot 70. Ring segments 90 are sequentially placed in the slot 70 of casing 15 until the slot 70 is filled with the design number of ring segment assemblies.
Any circumferential gap of unfilled slot space that remains after the last vane unit has been installed in the casing slot or the last ring segment has been inserted shall be filled by shims to maintain a design fit. The shims space the bases of the vane units or ring segments so that the engaging face of the last installed is within an allowable clearance with edge 140 of the casing. Failure to maintain design will result in vibration and excessive wear of components, possibly leading to failure during operation. At least one shim may be placed between the last and next-to-last space between stator vane units or ring segments or may be placed between the plurality of the stator vane units or the plurality of the ring segments.
FIG. 2 further shows a shim 130 spaced between two stator vane units 25. The shim 130 inserted between the engaging faces 60 of the stator vane units 25 are shaped generally conforming to the shape of the engaging face of the base 45 of the stator vane unit 25. The shim includes tabs 135 that engage (FIG. 3) the grooves 80 of the slots 70 of the casing 15, thereby helping to retain the shim 130 within the slot 70. Shims may be similarly employed to close gaps between segment rings, for those stages that employ the segment rings to hold stator vane units in place in the casing slots (not shown).
In the prior art, with the vane units and the shims moving because of aerodynamic forces on the airfoils, the tabs 135 wear away and the shims 150 can protrude into the flow path as seen in FIG. 5. FIG. 5 illustrates a sectional view of a casing with a shim protruding between stator vane units. Here, the engaging face 60 of the base 45 of the stator vane unit extends fully to the edge 140 of the casing 15 due to insertion of one or more shims in gaps 145 between the bases 45 of the stator vane units 25. A protruding shim 150 is shown that has partially protruded out of the gap 145 between the bases. Protruding shims can cause rotating blade stimulation and flow blockage. In addition, the shims can work their way totally out of the slot 70 in the casing 15 and enter into the air stream and cause blade foreign object damage (FOD) on downstream blades and vanes.
FIG. 6 illustrates protrusion of two shims 150 from the gap 145 between the bases 45 of two stator vane units 25. Attempts have been made to fix shims, in place and in different ways, in order to overcome this problem. FIG. 7 illustrates an existing method for retaining shims in place on stator vane units using drive pins. In this process, holes 125 must be drilled through the shim 130 and into at least one location on the engaging face 60 of a base 45 of the stator vane unit 25. Each stator vane unit type has different locations at which the holes must be drilled. The method also requires the added hardware of the drive pins. Anderson et al. (U.S. Pat. No. 6,984,108) addresses shims for stator vanes in gas turbine compressors. Anderson et al. attempts to prevent repositioning or release of shims into the turbine flow stream by a series of dowel pieces, positioned between adjacent bases for stator vanes. The dowel pieces fit into recesses in the adjacent base sections of the stator vanes. The dowel pieces are spring loaded and run through a hole in the center of the shim, thereby maintaining the shim in place. However, the spring loaded dowel pieces entail complexity with inherent potential for failure.
Accordingly, there is a need to provide a simple method for retaining shims on stator vane units and stator ring segments. The method should preferentially be simple and minimize requirements for additional hardware